There has been progress in the operation of telemetry systems in the far infrared wavelength region by use of p-Ge mode-locked lasers. Mode-locked lasers are well known as seen from numerous United States Patent Nos. including: U.S. Pat. No. 4,314,210 entitled Mode-Locking And Chirping System For Lasers; U.S. Pat. No. 4,375,684 entitled Laser Mode locking, Q-Switching And Dumping System; and U.S. Pat. No. 4,504,950 entitled Tunable Graded Rod Laser Assembly.
Mode-locked lasers have also been widely used for optical data transmission as seen in Publications including: Zhu, K. O. Nyairo and I. H. White, xe2x80x9cDual-wavelength picosecond optical pulse generation using an actively mode-locked multichannel grating cavity laser,xe2x80x9d Photon. Tech. Lett., Vol.6, No.3, 348-351, 1994; C. L Wang and C-L Pan, xe2x80x9cDual-wavelength actively modelocked laser-diode array with all external grating-loaded cavityxe2x80x9d, Optics Lett., vol.19, 1456-1458, 1994; and U.S. Pat. No. 3,935,543 entitled Laser Mode Locking And Coupling Method And Apparatus. In this last referenced Patent, it teaches of mode locking by phase modulation using an intracavity electrooptic crystal and use of this crystal as a fast gate to allow single pulses, or groups of pulses, out of the laser cavity. This achieves digital pulse code modulation to create a binary representation of a data signal.
The use of an optical communication system in which pulse position modulation(ppm) is achieved with a mode-locked laser with a mode-locking frequency equal to the time slot frequency of the modulation is described in U.S. Pat. No. 4,584,720 entitled Optical Communication System Using Pulse Position Modulation. This approach selects single pulses of optical energy to output from the laser cavity from the evenly spaced train of pulses circulating within the cavity. Control over which pulse to output is made by external electronic counters driven by a requisite external clock operating at the time-slot frequency. The temporal position of the output pulse with respect to the external clock pulse takes only discrete values. By choosing different pulses to output, different discrete delays of the output pulses with respect to the clock pulse can be realized, allowing a representation of a data signal via digital pulse modulation. Both the transmitter and receiver require synchronous clocks. A further example of the disadvantageous requisite external clocks at both the transmitter and the receiver is shown in U.S. Pat. No. 5,539,761 wherein there is disclosed generating a mode-locked pulse stream from a semiconductor laser using an oscillating resonant tunneling diode (RTD) to periodically circuit to bias and mode-lock the laser. PPM is realized by phase shifting the RTD circuit oscillations with respect to an external clock. The difficulties in synchronizing mode-locked laser output to an external clock are discussed.
The first objective of the present invention is to provide an actively mode-locked laser with analog pulse position modulation.
The second object of this invention is to provide analog pulse position modulation photon generation from a harmonically mode-locked laser source.
The third object of this invention is to provide analog pulse position modulation of far infrared photon generation from a harmonically mode-locked monocrystalline p-Ge laser diode.
The fourth objective is to is to develop a harmonically mode-locked laser source that will be appropriate for transmitting analog pulse position modulated pulses of optical energy and in particular far infrared radiation for advanced telecommunications and signal processing applications.
In a preferred embodiment of the invention described as analog pulse position modulation in harmonically mode-locked lasers, the laser must: have an active gain medium, whose gain changes as a function of a controllable parameter, and the gain-vs-parameter curve must display a maximum at some accessible value of the parameter; be harmonically and actively mode-locked by fast periodic changes of the controllable parameter at some even harmonic of the cavity round trip time resulting in a corresponding modulation of the gain; and, have means to impose a slow variation of the controllable parameter simultaneous to its fast periodic changes in such a way that the fast periodic gain change can be continuously shifted with respect to the peak of the gain-vs-parameter curve. The net result of the invention is a set (two, or a multiple of two) of interleaved, periodic, output pulse trains of optical energy whose relative temporal position (not frequency) can be varied as a function of the controllable parameter. This is an embodiment of pulse position modulation (PPM) without requirement of an external clock at the transmitter or receiver (a current disadvantage of present systems), and one in which the relative position of the interleaved pulse trains can be varied continuously rather than in discrete jumps.
Further objects and advantages of this invention will be apparent from the following detailed description of a presently preferred embodiment, which is illustrated schematically in the accompanying drawings and its embodiment as a transmitter.